Optokinetic nystagmus (OKN) is the manifestation of a visually guided reflex which operates to minimize retinal image motion. From the point of view of visual control systems, the slow phase of OKN is the important feature and is similar to foveal following and image stabilizing reflexes in man and other animals. The rabbit provides an excellent experimental model for studying the physiological mechanism of OKN because it has only this one class of visually controlled eye movement and a great deal is known about the characteristics of visual neurons throughout its visual pathway. The retinal direction-selective ganglion cells are thought to provide the error signals for the OKN system, and it now appears that these signals are going to the pretectal nuclei. It is therefore important to define the properties of these pretectal direction-selective cells and especially to see how they behave during open and closed loop optokinetic nystagmus. This will be done by recording action potentials from pretectal neurons with microelectrodes and comparing the firing frequency during optokinetic nystagmus with the actual eye movement velocity. The pretectal cells exhibit important features of the retinal direction-selective inputs, but are dissimilar in many respects due to the particular combination of retinal inputs made. We will attempt to specify the precise nature of input interaction at the pretactal level by comparing response properties of pretectal and retinal direction-selective cells. Autoradiographic techniques will be used to define the anatomical substrate of the OKN control pathway. The problems are the exact location and extent of the retinal innervation of the pretectum, and the location of efferent connections of the pretectum to other brain regions.